This invention relates to web dryers which are used in the manufacture of coated paper, film and foil and related processes such as printing.
Floater dryers are preferred for many web drying processes because they permit the web to be transported on a cushion of heated air such that it has no physical contact with any solid member such as a conveyer or roll until its surface is dry or cured. The air cushion provides support while drying the web. Furthermore, the absence of mechanical support members for the web allows the heat for drying to be applied intimately and uniformly to both sides of the web simultaneously. In this way drying intensity can be very high if desired.
The technology of floater drying has experienced substantial development in the past twenty years and certain important and desirable features have been discovered and quantified. Two basic types of nozzles have evolved, a single slot nozzle and a double slot impingement nozzle.
One of these nozzles, the single slot, nozzle 101 is described in U.S. Pat. No. 3,587,177 and is illustrated in FIG. 1. A plurality of these nozzles arranged in staggered formation on each side of the web 11 constitute a dryer. Heated air emerges from a single slot 103 and is turned around a curved surface to flow parallel to the travel direction of the web. The nozzle 101 creates what is known as the "Coanda effect" wherein the air does not impinge directly into the web and is constrained between the web 11 and a parallel plate 105 for a nominal distance (50-150 mm) to achieve high heat transfer. The heated air flow then continues for a similar distance beyond the trailing edge of the plate as a free wall jet parallel to and adjacent to the web. Finally, as the air flow approaches the next nozzle in sequence, it turns and flows away in the space between the nozzles.
This single slot nozzle 101 which creates the "Coanda effect" has seen extensive use worldwide. The single slot nozzle 101 provides high heat transfer which is uniform across the machine and fairly uniform in the direction of web movement. Because of the parallel direction of the air flow and web movement, the heat transfer can be further augmented by passing the web through the dryer such that it flows counterflow to the direction of the air. The local uniformity of heat transfer and consequent drying has beneficial effects to the quality of certain products and coatings dried on this type of machine. Since air flows are unidirectional, interacting streams of air are avoided which has benefits to cross-machine flow uniformity and web stability.
With the single slot nozzle 101, there is no positive pressure pad between the parallel plate 105 and the web 11. As a result, the web 11 travels through the dryer in a flat plane at a distance from the plate of about 2.5 times the width of the slot. Accurate alignment and parallelism of the nozzles 101 is required to avoid web 11 flutter at low tensions. At high tensions, webs have a tendency to curl at the edges and develop longitudinal wrinkles. When this occurs the possibility of contact between the web 11 and nozzles 101 is high. Thus, this type of nozzle 101 has limitations in some kinds of drying situations.
The principal alternative type of nozzle, the double slot impingement nozzle 107, is described in U.S. Pat. No. 3,873,013 and is illustrated in FIG. 2. This double slot impingement nozzle incorporates two slots 109 which blow air normal to the web 11. In this manner, a pocket of air at positive pressure is entrapped between the jets. A major portion of the air flow from the jets impinges against the web and flows away from both slots 109 on the nozzle 107. Some of this air rebounds directly away from the web 11 and some flows along the web 11 until it meets the corresponding stream from the adjacent nozzle. Heat transfer with this double slot nozzle 107 is comparable on average to the parallel flow type of nozzle under the same fan power conditions; however, there is much variability in heat transfer in the machine direction. In the immediate vicinity of the impinging jets, heat transfer is very high, but between each jet in the pair on the nozzle and in the region between the nozzles, it is quite low. For sensitive products, the high impingement heat transfer of this nozzle can cause quality problems. Interaction of the exiting streams of air between the nozzles can introduce web instability if the nozzles are placed too close together.
A very important feature of this double slot impingement type of nozzle is the positive pressure pad 111 formed between the impingement jets. Not only does this tend to keep the web 11 away from spurious contact with the nozzle 107, the staggered arrangement on each side of the web imparts an undulating motion to the web in the machine direction something like a sine wave. This corrugation effect gives the web some physical stiffness in the cross-machine direction which strongly resists tendencies to curl at the edges and to form wrinkles. This important feature of the double slot impingement nozzle also renders it less sensitive to dimensional accuracy in the positioning and alignment of the nozzles.
The pattern of pressure pads formed by the double slot impingement nozzle as arranged in a typical dryer is illustrated in FIG. 3 with pressure profile 113 and nozzle 107. It is characterized by the large spikes opposite the slots which are caused by stagnation of the air velocity at the web, a generally uniform elevated pressure between the spikes and a region to each side of the pressure pad where there is essentially no positive pressure.
The effect on the web of such a pattern of pressure pads is illustrated in FIG. 4 which also shows the local relationship between the pressure, the web tension and the radius of curvature of the web. For a local incremental region of constant pressure, the following equation applies: ##EQU1## where R is the radius of curvature, T is the web tension and P is the local pressure applied to the web. If P is zero, the radius of curvature is infinite which mathematically indicates that the sheet will be flat. If P is constant, the radius of curvature is a circular arc.
FIG. 5, FIG. 6, and FIG. 7 show the variation in web curvature for three different nozzle assemblies. FIG. 5 shows that the single slot nozzle causes the web to form a jagged undulation wave. Although the web undulates it has no curvature and therefore can curl locally. A double impingement nozzle applies pressure to the web over a finite distance b as shown in FIG. 6. Thus, ignoring the local effect of the spikes shown in FIG. 3, the generally constant pressure region will produce circular arc curvature over the pressure region with generally flat segments between them. This is a much better arrangement than is shown in FIG. 5 but the segments of the web having no curvature are still subject to local curl.
FIG. 7 shows that if the pressure region is made to be equal to half the undulation wave length, curvature is obtained throughout the length of the web. This is the objective condition for maximum resistance to curl. To achieve this with the double impingement nozzle requires that they be spaced on a pitch that is exactly twice the nozzle length dimension in the direction of the web movement. As discussed earlier, double impingement nozzles cannot be placed close together because of flow instabilities associated with the exiting flows meeting between the nozzles.
Another nozzle for obtaining a positive pressure pad with a parallel flow is described in U.S. Pat. No. 4,414,757. This nozzle modifies the basic Coanda type parallel unidirectional flow nozzle (FIG. 1) to produce a positive pressure pad without impingement of air against the web. This nozzle is herein termed the modified double slot nozzle. Extensive experimental work has shown that this technique can produce a pressure pad that is longer in the machine direction than the nozzle. It has no high spikes of pressure and can be configured, through proper selection of the design dimensions, to yield a web undulation pattern that maintains continuous curvature along the entire machine.
This modified double slot nozzle can provide pressure pad forces that are greater than those obtainable with the double impingement nozzle at the same conditions of flow and heat transfer. Furthermore, it retains the flow uniformity advantages of the unidirectional parallel flow nozzle and improves upon its heat transfer uniformity. The dimensional relationships obtained from the experimental investigation constitute the subject of the present invention.
The pressure level of the pressure pad shown in FIG. 9 is governed by the nozzle spacing which influences the kinetic pressure of the carry-over flow 5 and by the relative sizes of the primary jet 1 and the secondary jet 6. Processing difficulties may arise where there is a low or no pressure region which will allow the web to curl at the edges or to form wrinkles. The problem is further complicated by the fact that the nozzle spacing in a dryer will vary depending on the maximum drying rate required and the optimization of cost. In accordance with the present invention, the modified double slot nozzle is used to maximum advantage by optimizing the relationships of the spacing between the nozzles and the nozzle lengths in the machine direction.
If the size of secondary jet on the nozzle is too large in relation to the size of the primary jet, the Coanda effect will break down and the nozzle will become a skewed double impingement nozzle. As the secondary jet decreases in size, the pressure pad becomes weaker until at a secondary jet size of zero, the nozzle degenerates to a conventional parallel flow Coanda nozzle 101 as shown in FIG. 1.